1. Field of the Invention
The present invention relates to a zoom lens and, more particularly though not exclusively, to a zoom lens suitable as a photographic lens that can be used in image pickup apparatuses.
2. Description of the Related Art
In recent years, technologies for high resolution in image sensors, which can be used in image pickup apparatuses such as digital cameras, have been advanced. Photographic lenses, which can be used in such image pickup apparatuses, can be high-zoom-ratio zoom lenses.
Additionally, the photographic lenses require not only corrections for monochromatic aberration such as spherical aberration and coma aberration but also sufficient corrections for chromatic aberration that may occur when a white light source is used for imaging and that affects chromatic blurs and image resolutions.
In particular, a high-zoom-ratio zoom lens, whose focal length on the telephoto side is long, is useful to reduce a second-order spectrum in addition to first-order achromatism as corrections for chromatic aberration.
A method of using an extraordinary partial dispersion material as an optical material to correct chromatic aberration is discussed in U.S. Pat. Nos. 5,132,848 and 6,594,087.
Also, an optical system using a liquid material exhibiting optical properties of a high dispersion and an extraordinary partial dispersion has been discussed (see U.S. Pat. No. 4,913,535).
Additionally, a zoom lens in which color aberration on the telephoto side is corrected by utilizing a dispersion characteristic of an optical element obtained by dispersing fine indium tin oxide (ITO) particles in a resin has been discussed (see U.S. Pat. No. 7,088,521).
According to the conventional method of correcting chromatic aberration by disposing an extraordinary partial dispersion material, such as a fluorite, in an optical path, if a lens element made of this material does not have an appropriate refractive power or is not disposed at an appropriate location, even when chromatic aberration can be corrected at one of the wide-angle end and the telephoto end, chromatic aberration may be considerably deteriorated at the other of the wide-angle end and the telephoto end.
In a case where a liquid material having an achromatic effect is used, chromatic aberration can easily be corrected. However, a structure adapted to seal the liquid material is needed. A process for manufacturing optical systems tends to be complex. Additionally, a refractive index and a dispersion characteristic of the liquid material vary due to changes in temperature. Thus, it is difficult to maintain good environmental resistance of the optical system.